The following U.S. Patents are believed to represent the current state of the art:
6,913,017; 6,837,238; 6,655,385; 6,439,234; 6,422,240; 5,794,619;5,740,799; 5,495,848; 5,375,593; 5,335,656; 5,269,296; 5,046,491;4,572,177; 4,367,735; 4,156,426; 4,151,843; 4,106,505 and 2,693,800.